The present invention relates to a method for producing piezoelectric ceramics and, more particularly, to a method for producing piezoelectric ceramics of a system Pb(Sn.sub..alpha. Sb.sub.1-.alpha.)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 with .alpha. ranging from 1/4 to 3/4.
There are piezoelectric ceramics of a barium titanate system (BaTiO.sub.3), or lead zirconate titanate (Pb(Zr,Ti)O.sub.3) system, or lead titanate (PbTiO.sub.3) system which are now being manufactured and used in the electronic fields as electronic parts such as acoustic surface wave elements including acoustic surface wave filters, acoustic surface wave delay lines and acoustic surface wave discriminators; ceramic filters; ceramic resonators; ceramic vibrators; piezoelectric ignition elements; or piezoelectric ceramic transformers.
Such piezoelectric ceramics have generally been manufactured in the following processes: Weighing--Wet-Mixing--Drying--Calcination--Wet-Crushing (with binder)--Drying--Granulation--Forming--Firing. The most important process affecting the quality of the final products is firing which is effected in air. In order to prevent the evaporation of lead oxide from the composition during firing, it is the usual practice to fire forming bodies in a closed saggar made of a material which is unreactive to PbO and has no permeability of PbO, such as alumina or magnesia. In addition, PbO powder or mixed powder of PbO and ZrO.sub.2 is put in the sagger to form a lead oxide atmosphere surrounding the forming bodies.
However, the piezoelectric ceramics produced by the above normal air sintering method have relatively high porosity, large average pore size ranging from 5 to 15 microns. Because of their relatively high porosity and large average pore size, their regular use is impossible for some specific applications. For example, when a ceramic material is applied to acoustic surface wave filters having a general construction of FIG. 1 and comprising a piezoelectric ceramic substrate 1 and interdigital electrodes 2 and 3 formed thereon, the porosity and pore size must be small as much as possible since the width a of the electrodes decreases with the increase of operating frequencies of the filters. Assuming that the acoustic surface wave velocity is 2400 m/sec, a filter designed to be operated at a frequency of 58 MHz is required to have interdigital electrodes with a width a of about 10 microns since the wave length is about 41 microns. When split electrodes 4 shown in FIG. 1 are used to restrain the Triple transit Echo (Ref: PROPERTIES OF SPLIT-CONNECTED AND SPLIT-ISOLATED MULTISTRIP COUPLER, A. J. Devris et al. "1975 Ultrasonics Symposium Proceedings" IEEE cat. #75 CHO 994-4SU), they are required to have a width 4a of about 5 microns. Accordingly, if the piezoelectric ceramics used have high porosity, or if the pores connected to the surface of the ceramics are larger than 2 microns, the pores would cause the disconnection of the electrodes, in particular the split electrodes 4, thus making it difficult to produce acoustic surface wave filters for very high frequency applications.
When the piezoelectric ceramics with high porosity are applied to ceramic vibrators, ceramic filters, and the like, it is impossible to obtain the products with high quality because of the low strength of the ceramics. In the vibrators, it is the usual practice to apply a high voltage to obtain large amplitudes of the vibration. In this case, if the ceramics have a strain strength lower than the strain stress which occurs due to the vibration of the ceramics, it results in the breakdown of the ceramics.
In addition, in preparing miniaturized, ladder type filters which comprises a pair of resonators (a) and (b) as shown in FIG. 2, it is the usual practice to reduce the thickness of ceramic bodies for the 1st or 2nd resonator (a) or (b) to make the capacitance ratio between them large. Since, in the ladder type filters, there is a regular relation between the thickness and the mechanical strength of the ceramics, the ceramics with low strength make it difficult to keep the highest quality level of the product. In the resonators for high temperature applications, designed to be operated at temperatures ranging from 100.degree. to 200.degree. C., it is necessary to use ceramics with high thermal shock resistance in addition to the high strength. In this case, the pores present in the ceramics lead to serious problems. In the thickness expansion mode filters, its center frequency is inverse to the thickness of the ceramics so that the strength of ceramics has great influence on the quality of the product used in high frequencies. The most critical factors affecting on the strain strength and thermal shock resistance are the grain size and porosity, so that it is necessary to use ceramic materials with fine grain size and low porosity.
As a method for the fabrication of piezoelectric ceramics with low porosity, there have been proposed a hotpressing method and a hot-isostatic pressing method. However, both the methods have not been developed for mass-production, resulting in the considerable rise of manufacturing cost.
In addition to the above processes, there is a few specific processes using oxygen atmosphere sintering technique. For example, Gray S. Snow reported high-lead-oxide atmosphere sintering technique for producing transparent electrooptic lanthanum modified lead zirconate titanate ceramics, which is disclosed in "Fabrication of Transparent Electrooptic PLZT Ceramics by Atmosphere Sintering" (Journal of The American Ceramic Society, 56 (2) 91-96 (1973)). In this process, forming bodies of chemically prepared PLZT powder with excess PbO are sintered in an atmosphere containing oxygen and a very high partial pressure of lead oxide. The excess PbO is present as a liquid phase at the grain boundaries of the forming bodies at the sintering temperature, and this liquid phase enhances densification and pore removal by promoting mass transport along the grain boundaries to the pores. Although this process makes it possible to produce PLZT ceramics with low porosity, it has a serious disadvantage such that very large pores remain in the sintered ceramics because of the coalescence of small pores into large ones.
As described above, various attempts have been made to produce lead-containing piezoelectric ceramics with low porosity together with small grain size, but fully satisfactory results are not yet obtained.